I'd think you could get 1300+ with this bullet.I used a similar slug in the 44 mag for "sillywets" and had good results on chicken,pig& turkey targets. i used 2400 powder because it was handy.Of course the 445 case is a LOT bigger than the 44mag and you are on your own to find loads for it.As far as I KNOW ,It was designed for big slugs(250-300 grain)and is twisted faster than the 44.
My Hornady book shows a 200gr hp/xtp going 1700 from an 8" DanWesson using ww680 powder.Complete reloading for the T/C contender shows a 210JHC at 2200 fps from a14"T/c using ww680 and a220 grJFPequaling that velocity with about half a grain more 680.
Leading MAY be a problem if your lube isn't up to this kind of velocity.You could start with minimum loads for jacketed and work up to MAX loads to see what happens.Use a chronograph to see where problems start(leading ,tumbling,etc.)When you get a fall off in accuracy or to much leading ,try another powder and begin again. jh